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A top European security official warned yesterday that the threat of Isis (Islamic State) attacks is greater than previous assessments, underscoring calls for tighter security even as police widened the hunt for accomplices in the Brussels blasts.

Rob Wainwright, chief of Europol, said that the terrorist group has adopted a "more aggressive" posture toward Europe and that security authorities were focused on about 5000 suspects who had become radicalised in Europe and travelled to Syria to fight. Many have now returned.

"We are faced by a more dangerous, a more urgent security threat from so-called Islamic State," Wainwright told the BBC. "It threatens not just France and Belgium but a number of European countries at the same time ... It is certainly the most serious threat we have faced in at least a decade."

Wainwright spoke ahead of an emergency session of European security chiefs in Brussels. European leaders have been criticised for not acting more quickly to integrate security strategies, and they will be under pressure to produce results.

The bloodshed struck directly at "the liberty upon which the European project was built", said Belgium's Prime Minister, Charles Michel, in a speech to mark a national day of mourning.

Meanwhile, police pressed ahead with a manhunt for a suspected accomplice who is believed to have fled Tuesday's attack at the Brussels airport.

The French newspaper Le Monde and the Belgian broadcaster RTBF reported that video monitors had captured images of another possible accomplice, who is believed to have slipped away on the Brussels subway. The report could not be immediately confirmed.

Belgian police arrested six people in Brussels yesterday, while in France, a suspect who was plotting an attack has been arrested near Paris, officials said.

In a sign of the intense pressure on Belgian authorities following what are widely regarded as a host of security failures in the lead-up to Tuesday's attacks, the country's Interior and Justice ministers offered to resign yesterday, according to Belgian media reports. Both Interior Minister Jan Jambon and Justice Minister Koen Geens have come under criticism for their departments' inability to disrupt the terror cell before it struck, despite links between the Brussels plotters and the attackers in Paris last November.

The Brussels attackers had been on authorities' radar. One of the men who would become a suicide bomber, Khalid el-Bakraoui, 27, had even been subject to an international arrest warrant. The Belgian prosecutor's office said yesterday that the warrant was issued on December 11 last year and that he was wanted for using a false name to rent an apartment in the Belgian city of Charleroi that was used as a hideout for the Paris attackers.

On Thursday, authorities had suggested that Bakraoui and his older brother, Ibrahim, were spurred to carry out their attacks as security crackdowns and raids closed in.

Days before the attacks, counterterrorism police raided their Brussels safe houses. An ally who took part in a terrorist rampage in Paris last November was shot and captured by authorities. And Ibrahim el-Bakraoui, a 29-year-old Belgian with a long rap sheet, wrote that he did not want to wind up in a prison cell, Belgian federal prosecutor Frederic Van Leeuw said on Thursday.

The Bakraoui brothers were among three suicide bombers who struck on Tuesday, tearing apart a Brussels subway car and shattering the city's main airport terminal. At least 31 people were killed and 300 injured in the bloodiest attack on Belgian soil since World War II.

Bakraoui detonated a suitcase full of nails, screws and powerful explosives at the airport, killing himself in the process, Van Leeuw said. So did Isis bombmaker Najim Laachraoui, 24, who is also believed to have prepared explosives for the Paris attacks, according to an Arab intelligence official and a European intelligence official who spoke on the condition of anonymity because they were not authorised to discuss the matter publicly.

An unidentified man who left an even larger suitcase of explosives at the airport is believed to still be at large, he said. That suitcase did not detonate, sparing Belgium even more casualties.

Laachraoui's involvement draws the boldest line yet between the Paris attacks and those in Brussels. His DNA was found on explosives in the Paris attacks, and authorities believe he was versed in assembling powerful explosives from ingredients readily available. His participation in two attacks suggests that Isis is increasingly able to strike on European soil - although his death may also mean that he feared imminent capture by European authorities.

Belgian police patrols as people gathered to mourn for the victims of the bombings at the Place de la Bourse in the center of Brussels, Belgium. Photo /AP

Terrorism experts regard bombmakers, especially those trained in handling sensitive explosives, as among the most valuable and protected members of a terrorist organisation. It is highly unusual for them to participate in suicide attacks themselves.

Laachraoui's DNA was found in a Brussels apartment raided last week. The discovery of a militant cell there eventually led to the arrest of Salah Abdeslam last Saturday. Abdeslam is believed to have been involved in logistics for the Paris massacres, which claimed 130 lives.

Before Laachraoui allegedly learned to make bombs in Syria with Isis, he attended a Catholic school in the ethnically mixed Brussels neighbourhood of Schaerbeek.

"He was a good student," Veronica Pellegrini, the director of the Institut de la Sainte Famille d'Helmet, said of the bomber, who killed himself and dozens of others at the airport on Tuesday. Pellegrini said he spent six years at the school and studied humanities, and that he never had to fail and repeat a class.

In an interview, Pellegrini said the school never asks the students what religion they observe. And she said that the school has not heard from Laachraoui since he graduated in 2009.

The school's website describes its philosophy: "For us, school is a place of learning and a space for a common life. Each student acquires knowledge and skills, learns to live and to work with others." On another page, describing the school's philosophy, it quotes from the Bible: "We know that we have passed from death to life, because we love our brothers ... Do not love in word or speech but in deed and in truth."

It was another fragment in the lives of violent Islamic extremists who grew up in Brussels in a manner that gave no hints at what they would later become.

After a court hearing yesterday, Abdeslam's lawyer, Sven Mary, said the suspected terrorist is not fighting extradition to France, reversing his earlier position. In response to reporters' questions, the lawyer said the 26-year-old suspect had not known about the plans to attack Brussels and that his client "wants to leave for France as quickly as possible" so he can "explain himself".

Turkish President Recep Tayyip Erdogan said on Thursday that Turkey had deported the elder Bakraoui to Europe in July and warned European counterterrorism officials that it believed the man was a militant, suggesting a serious lapse by Belgian authorities.

The Reuters news agency, citing another Turkish government official, said Bakraoui was deported again in August after arriving in Antalya on Turkey's Mediterranean coast.

A photo, released to Turkish media, showed a police mug shot of Bakraoui - smiling and unshaven, wearing a dark T-shirt - prior to his deportation to the Netherlands in July.

Officials in Brussels on Thursday named a pair of brothers as two of the suicide bombers in Tuesday's bloody attacks in the Belgian capital. It was the third terrorist attack in Europe in a little over a year involving two male siblings, adding to questions about how individuals become radicalised and what authorities can do to stop it.

In January 2015, in an assault linked to al-Qaeda, brothers Said and Cherif Kouachi stormed a satirical publication in Paris, killing a dozen people. In November, attacks plotted by a cell of militants aligned with Isis (Islamic State), including brothers Brahim and Salah Abdeslam, rocked Paris. The former blew himself up at a Paris cafe; the latter was arrested last week in Brussels after months on the run.

The sibling phenomenon is not limited to Europe. In 2013, brothers Tamerlan and Dzhokhar Tsarnaev carried out the deadly bombings at the Boston Marathon.

"Jihad has become a family business," said Christina Schori Liang, a senior fellow at the Geneva Centre for Security Policy. "If one family member becomes radicalised, it is predictable that others will become militant as well."

According to Liang, more than a quarter of Westerners who have gone to fight in Syria have had a family connection to others involved in the armed Islamist cause, often a relative who also had gone to fight there.

It's not known whether any of the recent attackers were radicalised by their siblings or whether they happened to find similar appeal in the same ideology.

Like some of the others involved in recent attacks in Europe, the Abdeslam and the Bakraoui brothers (identified in the Brussels attacks) shared criminal records. All four had been convicted of robbery or other financially motivated crimes before they turned to militancy.

Faysal Itani, a scholar at the Atlantic Council in Washington, said siblings may be radicalised by proximity or may choose to pursue militant action with siblings because of the high level of trust required for such activity.

Abdelhamid Abaaoud, the mastermind of the Paris attacks, is thought to have taken his teenage brother to Syria to fight alongside the Islamic State. Abaaoud was killed during a standoff with police a few days after the November 13 string of shootings and explosions in Paris.

According to recent media reports, Abaaoud's brother Younes, now about 15, may be plotting revenge for his death from Syria.